1. Field of the Invention
The present invention concerns window glass equipped with an electroconductive coating based on metal oxides obtained by pyrolysis of powdered components, particularly for use as a windshield for automobiles.
In particular, the invention concerns such a coating which presents sufficiently low electrical resistance, while still allowing good transmission of light in the visible range, so that it can be used as heated window glass in an automobile.
2. Discussion of the Background Art
It is known from various prior patents that window glass can be manufactured, coated with conductive layers having light transmission properties greater than 75% or 70%, which, depending on legislation, which allows its use as window glass for automobiles, and having a resistance per square on the order of 10 ohms and even as low as 5 ohms.
Thus, the patent document EP-A-192 009 proposes window glass coated with a pyrolized ITO (indium tin oxide) layer obtained from powdered components, with a thickness of 1900 and 300 Angstroms, presenting a resistance per square which is 11 and 7.5 ohms, respectively, and a factor of light transmission of 83 and 82%, respectively.
The layer with a thickness of 300 Angstroms is redmauve in reflection, which has little chance of acceptance on the esthetic level, due to lack of harmony with the colors of car bodies.
Furthermore, these pyrolized layers present a slight speckling and a slight haze, which are not at all a problem for window glass used in buildings, but can be objected to for automobile window glass. Thus, the standards for automobile windshields require that the haze (percentage of diffused light) not be greater than 0.5%, which is less than what the eye can perceive with normal vision (2 to 3%) and than what is accepted for windows intended for buildings.
It should be noted that this prior patent EP 192 009 proposes production of window glass laminated with an ITO layer, but to facilitate the thermal reduction treatment of the ITO, it provides that the said thermal reduction treatment is carried out on the finished laminated window glass, with a burner, this burner having the advantage of heating the ITO layer rapidly to treat it, without having time to heat the structure of the window glass, the ITO layer being arranged on the exterior of the window glass for this purpose, directly in contact with the flame of the burner.
The patent document U.S. Pat. No. 4,490,227 proposes window glass coated with an ITO layer obtained by vacuum techniques, with a thickness on the order of 2800 Angstroms, transmission on the order of 78%, resistance per square approximately 7 to 10 ohms.
These layers under vacuum take long to produce and are expensive, and obtaining lower resistances increases the production time and the cost, on the one hand, and causes the risk of exceeding the limits of 75 or 70% transmission required for automobile window glass, on the other hand.
Patent document U.S. Pat. No. 4 655 811 discloses ITO layers deposited under vacuum with resistances per square down to 5 ohms with light transmission compatible with applications in automobiles. Production costs are high, production time is long, and increasing the thickness would increase those costs and production times even more, while reducing light transmission too low.
Also known is window glass coated with a stack of layers obtained by vacuum techniques, layers among which there is a layer of Ag surrounded by dielectric layers. This type of window glass has an electrical resistance greater than 5 ohms, similar to the preceding types already mentioned, and also these layers easily oxidize and degrade, particularly due to humidity. Furthermore, the sheets of glass coated with these stacks of layers containing an Ag layer require great precautions during lamination to produce laminated window glass, because the least spot of dirt, the least bit of dust, creates a visible defect.